CIRED Workshop - Lyon, 7>8 June 2010
Paper 0053
ESB’s ADOPTION OF SMART NEUTRAL TREATMENTS ON ITS 20 KV SYSTEM
Martin HAND
ESB – Ireland
martin.hand@esb.ie
ABSTRACT
ESB Networks has been upgrading its Medium Voltage
Network from 10kV to 20kV over the last 15 years. As a
result of the change in voltage level there has also been a
change in neutral treatment from isolated to resistance
earthed. Operational deficiencies of the 20kV resistance
earth system, specifically relating to customer hours lost
and customer interruptions have prompted an
investigation into alternative neutral treatments. As part
of this investigation two different neutral treatments are
being explored, namely: Peterson coil coupled with
enhanced control, and Faulted Phase Earthing (FPE)
through the use of a custom built controller. This unique
FPE system has been patented by the ESB [1].
It is hoped that these trial system will deliver enhanced
operational performance. In this regard it is planned to
gain sufficient experience of both trial systems in order to
form future policy with regards to the operation of the
20kV system in Ireland. The research and development of
the trial systems over the last 2 years has enabled the
building up of new bodies of knowledge around these
systems specifically relating to: testing, fault passage
indication devices, location of earth faults, applying live
faults for test purposes and event recording. This paper
details the development of each trial system and the
benefits associated with each system.
Neil MC DONAGH
ESB International – Ireland
neil.mcdonagh@esbi.ie
The trial systems built by ESB are:
Neutral Earthing via Peterson Coil complimented with
a range of earth fault management facilities.
Interchangeable resistance earthed/isolated system in
conjunction with Faulted Phase Earthing (FPE)
PETERSON COIL TRIAL SYSTEM
Background
Perhaps the most popular form of MV Neutral earth
treatment employed in Europe is Arc Suppression. In ESB
Arc Suppression has been in use on the 38kV subtransmission network since 1928. Internal research [3] had
suggested that Arc Suppression would be difficult to
implement in MV Networks were the level of asymmetry
can be as great as 70% as a result of significant lengths of
two phase network. Indeed this was the very premise that
sponsored the development of the unique FPE solution
outlined in this paper. Recent work has suggested the use
of a secondary damping resistor as both a solution to
asymmetry and an aid to fault hunting [4].
It was proposed in 2008 to implement a trial scheme at
38kV/20kV station feeding a 20kV radial network with
approximately 400 km of overhead network. The scheme
has been live since July ’09. Elements of the scheme are
described in detail in [5]. A simplified illustration of the
components relating to the system is shown in Figure 1.
INTRODUCTION
ESB’s 20kV system is at present operated as a resistance
earthed system, with 20Ω resistors on the neutrals of the
20kV windings of transformers. The existing system is
performing at a fault rate of 14.5 Faults (resulting in an
outage) per 100 km of Network per annum. The remaining
10kV (isolated neutral) operated Network is performing to
a fault rate of 7.95 Faults (resulting in an outage) per 100
km of Network per annum. The majority of faults are
Single Line to Ground (SLG) faults, which are
significantly influenced by the neutral treatment of the
system.
As a result of penalties for customer hours lost and
customer interruptions alternative methods of system
operation are being investigated. It is suggested that the
best performing systems for earth fault arc quenching
would be Compensated, High Impedance and Isolated in
that order [2]. It is therefore not difficult to see why a
change would be contemplated, as the existing system is
low impedance earthed .
Paper No 0053
Figure 1. Components of trial ASC system
Where: ASC-C Arc Suppression Coil Controller
PAW
CIF
DR
ER
PLC
EOR-D
Power Auxiliary Winding
Current Injection by Frequencies
Damping Resistor
Earth Resistor
Pulsing Controller
Earth Fault Detection
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CIRED Workshop - Lyon, 7>8 June 2010
Paper 0053
The trial system has afforded the opportunity to use some
smart functionality not currently employed on the older
38kV system.
a) The use of a DR in order to carry out resonance curve
shaping and wattmetric detection of faults
b) Pulsing of DR to increase fault detection sensitivity.
c) Current injection by two frequencies (non 50Hz) to
enable tuning without coil movement.
d) Individual feeder online capacitive current
measurements
e) Rapid control of ASC, tapping to near resonance point
during SLG fault conditions to minimize fault current
f) Fast recalculation of resonance point, coupled with
tapping of ASC following the switching of a feeder
g) Implementation of 4 parallel fault detection algorithms
h) 3G based telemetry device which allows remote
records and log file extraction, online setting, remote
firmware upgrades, remote ASC tuning, email and
SMS services, to remote desktop
been reduced as a result of the implementation of the trial
Peterson coil earthed system, and fault location times have
been reduced substantially as a result of the new systems
and procedures used for the location of faults.
Fault passage indicators and fault hunting
Risk assessment dictates that if an attempt is to be made to
sustain an SLG fault then the possible risks associated with
this would be mitigated by a precise system of quick fault
location. ESB are successfully finding faults on this trial
system using a combination of the following actions:
Location of faulted section of network. Stationary
Fault Passage Indicators (FPI), coupled with an inbuilt
communication system are used to locate the faulted
section of Network. This combined system is known as
the X-Net and is produced by FMC-Tech. This system
will successfully identify between which two nodes the
fault has occurred. The hardware installed at each node
is shown in Figure 2. The information available to the
operators, via web-link using an i-phone is shown in
Figure 3. A graphical view of the fault location is
shown in Figure 4. This system has proved extremely
useful and has preformed well to date. In a number of
instances during the testing cross country earth faults
developed. In these instances supply was lost to the
faulted feeder(s), however the X-Net system was still
able to identify the section(s) of faulted network. More
details of this system can be found in [6].
Figure 2 X-Net FPIs Hardware at each Node
Figure 3. Information Available to Operator
Location of Fault. Once the section of network has
been identified, a procedure has been developed to
locate the fault. Using the components and
functionality of the ASC system the DR and/or
Capacitor is pulsed. A portable fault passage locator
(pathfinder) is then used to ‘walk’ to the fault.
Figure 4 Graphical View of Fault Location
The number of customer outages as a result of faults has
Paper No 0053
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CIRED Workshop - Lyon, 7>8 June 2010
Paper 0053
FPE TRIAL SYSTEM
Background
The proposed system will operate as an interchangeable
high resistance earthed/isolated system in conjunction
with FPE, by the use of a custom designed Earth Fault
Controller (EFC). The system uses a 300Ω Neutral Earth
Resistor (NER), which is switched out during sustained
faults. This NER is required to restrict switching overvoltages. During a single line to ground fault, the faulted
phase will be earthed and transformer neutral will be
isolated, as shown in Figures 5 and 6. As the phase to
phase voltage is unchanged with the FPE applied, and load
is connected via delta windings, the supply voltage to
customers is maintained. This system has been patented by
the ESB [1].
than 3kΩ. The controller algorithms are designed using
Labview® graphical programming language, which is
produced by National Instruments. All the hardware
associated with the controller is also supplied by National
Instruments. The program was developed on a
conventional PC and installed on a stand alone industrial
computer which has real time capability. An illustration of
the system hardware is shown in Figure 7. All I/O
functions are channelled through a signal processing unit.
The EFC performs each of the following:
Measures individual phase currents on three feeders as
well as the neutral current, three phase voltages,
Operates digital outputs to control four switches within
the substation compound on the neural and three
phases.
Signals a number of alarms, which identify the nature
of the fault, which are connected to the SCADA
system.
Figure 5. System under normal operation
Figure 6. Operation under single phase fault
The operation of this particular FPE system is enabled by
the use of a custom built EFC that has the ability to detect
high impedance faults of up to 12kΩ. The EFC can also
successfully identify single pole switching events, which
have at times caused the mal-operation of existing
protection. As FPE involves the earthing of a faulted phase
during an SLG fault, it is ensured that the fault site is made
safer and that no customers are interrupted during the
fault. Pervious work details the algorithms associated with
the EFC [7], and testing of the new FPE system [8], which
will be discussed briefly in this publication.
Earth Fault Controller
The controller provides a robust method of detecting SLG
faults of up to 12kΩ, and can distinguish between high
resistance faults and single pole switching events. This is
an improvement on the existing system which has the
ability to detect single line to ground faults of not more
Paper No 0053
Figure 7. System Interface of EFC
The EFC works by closing the appropriate FPE switch as a
result of a significant change in neutral current. The EFC
then computes the change in the phasor values of voltages
and currents before during and after the FPE cycle. During
this analysis the controller specifically looks at the change
in zero sequence current to determine if there is an earth
fault on the system, further details may be found in [7].
Testing of this system has been carried out in a number of
ways: firstly by injecting of simulated currents and
voltages at the primaries of measurement current and
voltage transformers, and secondly by applying earth
faults to the system, and carrying out single pole switching
operations. The controller has preformed well using both
methods, further details may be found in [8].
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CIRED Workshop - Lyon, 7>8 June 2010
Paper 0053
CONCLUSIONS
The ESB sought to investigate alternatives to the exiting
method of neutral earthing and system operation of the
20kV distribution network Two different methods have
been identified and successfully implemented at trial sites
in Ireland. The trial systems are:
Neutral Earthing via Peterson Coil complimented with
a range of earth fault management facilities.
Interchangeable resistance earthed/isolated system in
conjunction with Faulted Phase Earthing (FPE)
Conclusions will be drawn separately for each system
Peterson Coil Trial System
The trial system using the person coil is operational since
July 2009. Operational experience gained from this system
suggests that there are less outages due to faults than the
20kV resistance earthed system. Fault location times have
been reduced substantially as a result of the new systems
and new procedures used to locate faults. The benefits of
this system include:
Improved Sensitivity: The ASC will treat all faults
irrespective of Ohmic value. Signalling of an EF is
triggered by Neutral Voltage Displacement of 30%.
Selectivity: Existing SEF protection may mal-operate due
to MV single pole switching on a long two phase spur. The
Peterson Coil system removes this restriction.
Fault Location: With the system, SLG have been tracked
down successfully within the 3 hour time limit.
Supply Continuity: From a technical standpoint no
customers need be interrupted during a single line to
ground fault. Risk assessment would dictate that for urban
networks safety risk can be mitigated via a tripping
strategy. The first eight months of operation have shown
the Network to be operating at a rate of 4 faults per
100KM of network per annum.
Fault Site Safety: The system normally operates at 5%
over-suppressed. For the purpose of fault site voltage
management the coil retunes to a net 2A away from
resonance within 5 seconds of permanent fault inception.
FPE Trial System
The trial system has been built and is still under test. It is
hoped that this system will become operational during
2010. Experience gained shows that the system works,
however it is more onerous on the system insulation than
the Peterson coil solution. Work previously carried out
suggests that it will provide a greater level of fault site
safety than the Peterson coil solution [3]. The benefits of
this system include:
Improved Sensitivity: Existing protection on the 20kV
system can identify a fault of about 3kΩ (~4A) while the
EFC can identify a High Impedance Faults (HIF) of up to
12kΩ (~1A) and even up to 16kΩ.
Paper No 0053
Selectivity: Existing SEF protection may mal-operate due
to MV single pole switching on a long two phase spur.
The controller using FPE can identify this event as not
being a fault
Fault Location: With the existing system, network
technicians may have no indication of the phase or feeder
on which a fault has occurred. The EFC will identify the
faulted feeder and phase thus saving time in fault location.
Supply Continuity: With the use of FPE on the 20kV
system no customers need be interrupted during a single
line to ground fault, until the fault is found and an
adequate plan is formulated how to remove the fault,
which may involve temporarily switching out customers.
Fault Site Safety: Previous work carried out by Tobin et
al [3] has shown that fault site GPR using the FPE is
generally lower than a similar fault on a system with a
neutral earthed via a Peterson coil.
AKNOWLEDGEMENTS
The authors would like to show their gratitude to :
ESB staff at Port Laois and Waterford for their
willingness to adapt to change, and their diligence in
learning how to operate these new systems
All in ESB Networks Asset Management and ESBI
High Voltage for their support throughout this project.
FMC-Tech, National Instruments, and UCAutomation
REFERENCES
[1] Electricity Supply Board, of Ireland, 1997, “Fault
Detection Apparatus and Method of Detecting Faults
in An Electrical Distribution Network”, Patent
Application Number S970641.
[2] M. Lethonan & T. Hakola “Neutral Earthing and
Power System Protection”
[3] N. Tobin, B. Brady, “Managing Earth Faults on
Distribution Networks”, Paper Presented to Institution
of Engineers of Ireland. (1997)
[4] P. Toman, M. Paar, J. Orsagova “Possible Solutions to
Problems of Voltage Asymmetry and Localization of
Failures in MV Compensated Networks.” Research
from project MSM0021630516, Ministry of
Education, Czech Republic, 2007.
[5] R. Voncina, I. Watson “Vloga Modernih Ozemljitvenihdusilk V Danasjih Elektroenergetskih Sistemih”,
Hoflerjevi dnevi 2007.
[6] http://www.fmc-tech.com/
[7] N. McDonagh W. Phang “Use of Faulted Phase
Earthing Using a Custom Built Earth fault Controller”,
10th International Conference on Development in
Power System Protection, IET, March 2010
[8] N. McDonagh “Testing of ESB’s 20kV Faulted Phase
Earthing System” PAC World, June 2010.
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